Redskins quarterback Robert Griffin III was throwing from a chair early in his recovery. / Richard Lipski, AP

by Gary Mihoces, USA TODAY Sports

by Gary Mihoces, USA TODAY Sports

Adrian Peterson's run through the NFL last season on a reconstructed knee is inspiration to others coming off similar surgeries. But doctors say it's no guarantee of a similar type and time of recovery .

"Adrian Peterson came back so well that in some ways it gives people somewhat unrealistic expectations," says Andrew Pearle, orthopedic surgeon at the Hospital for Special Surgery in New York.

The Minnesota Vikings running back returned less than nine months after surgery ACL and MCL surgery to rush for 2,097 yards, 13 shy of the single-season record, and lead his team the playoffs for which he was named the league's MVP.

"For every Adrian Peterson who comes back like he has, there are lots of athletes who struggle the first year," Pearle says. "It doesn't mean they're not working hard. â?¦ But that's an example of somebody who came back in a very, very remarkable way. We hope for recoveries like that. We don't always get it."

Multiple variables play roles. If the injured athlete has had the inner-knee ligament known as the ACL replaced by a graft, what other knee damage did they have? How well and how quickly do the important muscles of the thigh regain their strength?

But there first question is always how soon will they be back? Now, it's can they come back as Peterson did?

"It's such a hot subject because of Adrian's extraordinary success," says Leon Popovitz, orthopedic surgeon and co-founder of New York Bone & Joint Specialists.

But, he adds, "A lot of people end up returning six months to 12 months later and not being at their ideal, peak level until sometime the season afterwards, after the 12-month period."

Several high-profile athletes have sustained ACL injuries recently.

Recovering from knee reconstructions following injuries during the last football season are quarterback Robert Griffin III of the Washington Redskins and University of South Carolina running back Marcus Lattimore.

Lindsey Vonn, Olympic skiing champion, underwent surgery this week for knee damage sustained in a crash this month in Austria. Nerlens Noel, the University of Kentucky's freshman basketball sensation, went down this week with an ACL injury.

NBA All-Star guard Derrick Rose of the Chicago Bulls had his knee surgery last May. It remains uncertain when he'll be back.

"Every ACL injury is different. It takes some people longer. â?¦ (Rose) is clearly not ready, and they're not going to rush him back," says Pearle.

Severely torn ACLs are replaced by grafts, which can be patellar or hamstring tendons from the athlete's own body or tissue from a cadaver.

"I think the most important variable is what other injuries you have in addition to the ACL injury?" says Pearle.

"For example, somebody who tears up or loses a large portion of their meniscus (cartilage) can have a much more difficult time coming back. â?¦ There have been studies in the NFL that show that only 60%-70% of players actually make it back to the NFL after ACL reconstruction. â?¦ With some ACL injuries, there is such profound damage to the meniscus that we can't save it or we can't save a large portion of it.''

During the rehabilitation, the strength of the quadriceps muscles in the thigh is closely monitored. How that goes varies by athlete.

"Some of that is effort, and some of it is luck, and some of it is genetics," says Pearle.

"Some people after the surgery lose a lot of muscle tone and function. Even with amazing effort, they have a tough time getting their muscles rebuilt. â?¦ Some people can build up their quadriceps very quickly. â?¦ There are some people who are neuro-muscular kinds of geniuses."

Pearle adds, "Adrian Peterson had to be a guy like that."

Popovitz says there also are intangibles.

"Confidence is a huge intangible in any sport," says Popovitz. "If you have that confidence in yourself that you have that full strength, then you will be at your top level. You're not going to perform at your top level until you get that intangible.''

He sees that in Peterson. "Adrian is so special ... Not only is he physically, just exceptionally gifted, he has the mental makeup that will give him confidence and extreme concentration," Popovitz says.

After hearing accounts that Griffin was throwing from a chair during his early recovery, Popovitz sees the same in him. "It's not different from the young quarterback at Washington, RGIII, that he has that very same makeup," says Popovitz. "It's one of the reasons why these people can really just flourish (post-surgery)."

It's not the same for everybody.

"Everyone is individual," says Popovitz. "Professional athletes particularly, they're a different breed. â?¦ When it comes to professional players, sometimes it's a bit more subjective for the doctor or the surgeon."